I figured out a way to simulate the motion blur in Blender in a way that provides more accurate and photorealistic results, not to mention the nice price tag of free. This is now my preferred method.

A New Technique

The gist of it is to project the backplate onto a curved plane acting as a backdrop in a way similar to camera mapping, and physically moving the vehicle on that plane. The camera is parented to the car so that the two are always relative to each other, but the landscape “moves” as the car drives forward.

The backplate is then rendered with motion blur, and composited with the other passes as usual. In effect these steps simulate a real car photography rig — the kind with a long rod attached to the car with suction cups and the camera at the opposite end. The nice thing about this technique is that it essentially eliminates the guesswork of vector blurring since it creates a reasonably accurate simulation of the real scene, leading to a more believable result.

And here’s a side-by-side comparaison showing my best guess using Bleex, and the result with the new technique directly in Blender. Note that there's no reason Bleex couldn't give the same motion blur, it's just that it's all guesswork and experience whereas the Blender simulation provides an easier workflow. It also gives a slightly different texture to the blur, which to my eyes looks more natural: